The computer program for causing the PLC to operate is created using a programming apparatus. When a large-scale computer program is developed using this programming apparatus, for improvement of productivity and maintainability, programming employing labels (or tags or variables) is performed. For example, the IEC61131 standard by International Electrotechnical Commission regulates a high-level program language for a PLC employing labels.
The PLC cannot directly execute the computer program employing labels. Therefore, the programming apparatus converts the computer program employing labels into a program code executable by the PLC and downloads the converted program code to the PLC. Consequently, the PLC can execute the computer program created using labels.
The computer program is executed in the PLC and generates and updates input and output data for controlling external equipments such as industrial equipments and intermediate data for calculating the input and output data in a memory included in the PLC. In the following explanation, the input and output data and the intermediate data are collectively referred to as device data. This device data in the PLC is sometimes monitored and operated using a display or monitoring software on a personal computer. In the following explanation, the display is not limited to only a programmable display and includes an apparatus that displays device data in the PLC such as a personal computer that causes the monitoring software to operate. In the past, when screen data to be displayed on the display is created using a drawing apparatus, there is a technology that can use a label name for designation of this device data to be monitored and operated. This technology is hereinafter referred to as related art 1.
According to the related art 1, when the programming apparatus converts the computer program employing labels into the program code executable by the PLC, the programming apparatus allocates memory addresses corresponding to data types to the labels. According to this processing, label information formed by label-data type-memory address is generated. The drawing apparatus captures this label information, converts labels allocated to various components on screen data into memory addresses (device addresses) of an actual PLC data memory by referring to the captured label information, and downloads the memory addresses to the display. The display can acquire the device data to be monitored and operated from the memory addresses in the PLC allocated to the labels according to the label information.
Patent Literature 1 discloses a technology for utilizing ID information given to data for the purpose of realizing efficiency of data transmission and reception between a PLC and a programming apparatus. According to the technology disclosed in Patent Literature 1, the ID information is updated every time data of a computer program of the PLC is corrected. Because it is difficult to transmit and receive only the corrected data between the PLC and the programming apparatus, a transmission source and a reception destination compare the ID information and determine whether the data is corrected. As such ID information, data final correction date and time (a time stamp) is often used.